Exclude elements with no composited layer from touch event rectangle calculation.

Some HTML pages exclude their elements by changing their style
dynamically, e.g. after screen resize. Then some element might loose
their owner renderer and the composited layer. But later some calcu-
lations assert that this layer exists.

This fix excludes the elements from the walk in the function
accumulateDocumentTouchEventTargetRects() and prevents the assertion
from happening.

The test case crashes in the current code (before the fix) in the debug
build only.

BUG=402148

TEST=LayoutTests/fast/events/touch/compositor-touch-hit-rects-iframe-disappears.html

Review URL: https://codereview.chromium.org/502263002

git-svn-id: svn://svn.chromium.org/blink/trunk@181126 bbb929c8-8fbe-4397-9dbb-9b2b20218538